Edward had no idea how much she wanted to keep the tart for herself, but she was trying to be nice!  She really should save it for later, but she's already offered some to her friend, and she really did want him to have some, maybe more than she wanted it for herself.

"It's not mine, its ours.  If I want more, I'll pester my brother for a bite of his.  After all, the strawberry is very good - the best flavor I think.  Have you ever had it before?  Here, have a bite at least, Edward, then I'll wrap it up.  Please?"

It was the same trick her brother used to use on her to get her to eat when she was much younger, and maybe Edward would see right through her, but she'll be darned if she didn't try!